This invention is related to tonneau covers and related support structures for enclosing the cargo area of a vehicle. In particular it relates to a cover structure in which the cover, itself, has a resilient extrusion stitched along at least one side edge of the cover to interlock with a complementary rail, usually extruded of aluminum, mounted along the top edge of cargo box. The rail includes at least one outwardly directed lip to be engaged by an inwardly turned lip of the resilient extrusion. In the case of a "J" shaped extrusion, the inwardly turned, resilient lip is adapted to unwind like a clock spring to accommodate variations in the dimensions of the cover and of the cargo box due, primarily, to changes in temperature of the environment. Normally the cover is attached to all four walls defining the entire perimeter of the cargo box and is cut to fit the box so as to be stretched relatively tight across it. Such covers have several problems. For example, the cargo box sidewalls of commercially-available pick-up trucks frequently vary in width because of manufacturing tolerances. Thus, a cover adapted to accommodate a class of vehicles frequently will be loose on some vehicles and tighter on others because the means for connecting the cover to the sidewalls do not accommodate such dimensional variations.
Another problem is that the tonneau cover is usually formed of a flexible material that can be somewhat stretched, however, some materials that are particularly suited for use as a tonneau cover do not stretch equally in both directions. We have found it desirable to fit covers so that they stretch very little in the longitudinal direction of the truck and stretch to a greater degree in the transverse direction. However, in cold weather, the material loses much of its flexibility and becomes stiff, and thus, hard to install and prone to tear. It also contracts in cold weather. During warmer weather, the flexible material becomes very soft and flexible, and thus, difficult to tension.
Further, commercially-available fastening devices for connecting a tonneau cover to a cargo box are difficult to attach and in some cases require drilling several holes in the cargo box sidewall. This detracts from the ultimate resale value of the vehicle and promotes corrosion of the metal in the area of the holes. In other cases, the existing attachment structures are relatively complicated and expensive to install and do not include convenient means for holding the cover so that it is taut but not over-tensioned at any time of the year.